Method and system for a reference signal (RS) timing loop for OFDM symbol synchronization and tracking

ABSTRACT

Aspects of a method and system for a reference signal (RS) timing loop for OFDM symbol synchronization and tracking may include tracking symbol timing in an Orthogonal Frequency Division Multiplexing (OFDM) signal based on at least a reference symbol set. A receiver timing may be adjusted based on at least the symbol timing. The symbol timing may be tracked by generating an output signal as a function of a guard time Δt g  in a phase discrimination feedback loop. The reference symbol (RS) set may be generated in an RS extraction module or circuit, from at least a fast Fourier transform of the received OFDM signal. The receiver timing may be coarsely adjusted and then finely adjusted. The coarse receiver timing adjustment may be based on processing at least a primary synchronization signal and a secondary synchronization signal.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.12/184,383, filed on Aug. 1, 2008, now U.S. Pat. No. 8,174,958, andmakes reference to:

-   U.S. application Ser. No. 12/184,353, filed on Aug. 1, 2008; and-   U.S. application Ser. No. 12/184,410, filed on Aug. 1, 2008.

Each of the above referenced applications is hereby incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing forcommunication systems. More specifically, certain embodiments of theinvention relate to a method and system for a reference signal (RS)timing loop for OFDM symbol synchronization and tracking.

BACKGROUND OF THE INVENTION

Mobile communications have changed the way people communicate and mobilephones have been transformed from a luxury item to an essential part ofevery day life. The use of mobile phones is today dictated by socialsituations, rather than hampered by location or technology. While voiceconnections fulfill the basic need to communicate, and mobile voiceconnections continue to filter even further into the fabric of every daylife, the mobile Internet is the next step in the mobile communicationrevolution. The mobile Internet is poised to become a common source ofeveryday information, and easy, versatile mobile access to this datawill be taken for granted.

Third generation (3G) cellular networks have been specifically designedto fulfill these future demands of the mobile Internet. As theseservices grow in popularity and usage, factors such as cost efficientoptimization of network capacity and quality of service (QoS) willbecome even more essential to cellular operators than it is today. Thesefactors may be achieved with careful network planning and operation,improvements in transmission methods, and advances in receivertechniques. To this end, carriers need technologies that will allow themto increase throughput and, in turn, offer advanced QoS capabilities andspeeds that rival those delivered by cable modem and/or DSL serviceproviders. Recently, advances in multiple antenna technology and otherphysical layer technologies have started to significantly increaseavailable communications data rates.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A method and/or system for reference signal (RS) timing loop for OFDMsymbol synchronization and tracking, substantially as shown in and/ordescribed in connection with at least one of the figures, as set forthmore completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention.

FIG. 2 is a diagram illustrating an exemplary OFDM symbol stream, inaccordance with an embodiment of the invention.

FIG. 3A is a diagram of an exemplary OFDM timing acquisition andtracking system, in accordance with an embodiment of the invention.

FIG. 3B is a diagram illustrating phase offsets for exemplary timingoffsets, in accordance with various embodiments of the invention.

FIG. 3C is a diagram illustrating phase offsets for exemplary timingoffsets by subcarrier, in accordance with various embodiments of theinvention.

FIG. 4 is a flow chart illustrating an exemplary timing acquisition andtracking process, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor RS timing loop for OFDM symbol synchronization and tracking. Aspectsof the method and system for a reference signal (RS) timing loop forOFDM symbol synchronization and tracking may comprise tracking symboltiming in an Orthogonal Frequency Division Multiplexing (OFDM) signalbased on at least a reference symbol set. A receiver timing may beadjusted based on at least the symbol timing.

The symbol timing may be tracked by generating an output signal as afunction of a guard time Δt_(g) in a phase discrimination feedback loop.The reference symbol (RS) set may be generated in an RS extractionmodule or circuit, from at least a fast Fourier transform of thereceived OFDM signal. The receiver timing may be coarsely adjusted andthen finely adjusted. The coarse receiver timing adjustment may beachieved based on processing at least a primary synchronization signaland a secondary synchronization signal. The reference symbol set maycomprise a plurality of time-frequency slots. The plurality oftime-frequency slots may change according to a frequency shift and PNsequence that may modulate the reference symbols. The PN generatedsequences may be determined by base station identifier. This basestation identifier may be determined by the primary synchronizationsignal (PSS) and secondary synchronization signal (SSS). The OFDM signalmay conform to a Universal Mobile Telecommunications Standards (UMTS)long-term evolution (LTE) signal. The adjustment of the receiver timingmay be controlled via a receiver timing generator.

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention. Referring to FIG. 1A,there is shown a building 140 such as a house or office, a mobileterminal 142, a factory 124, a base station 126, a car 128, andcommunication paths 130, 132 and 134.

The base station 126 and the mobile terminal 142 may comprise suitablelogic, circuitry and/or code that may be enabled to generate and processMIMO communication signals.

Wireless communications between the base station 126 and the mobileterminal 142 may take place over a wireless channel. The wirelesschannel may comprise a plurality of communication paths, for example,the communication paths 130, 132 and 134. The wireless channel maychange dynamically as the mobile terminal 142 and/or the car 128 moves.In some cases, the mobile terminal 142 may be in line-of-sight (LOS) ofthe base station 126. In other instances, there may not be a directline-of-sight between the mobile terminal 142 and the base station 126and the radio signals may travel as reflected communication pathsbetween the communicating entities, as illustrated by the exemplarycommunication paths 130, 132 and 134. The radio signals may be reflectedby man-made structures like the building 140, the factory 124 or the car128, or by natural obstacles like hills. Such a system may be referredto as a non-line-of-sight (NLOS) communications system.

Signals communication by the communication system may comprise both LOSand NLOS signal components. If a LOS signal component is present, it maybe much stronger than NLOS signal components. In some communicationsystems, the NLOS signal components may create interference and reducethe receiver performance. This may be referred to as multipathinterference. The communication paths 130, 132 and 134, for example, mayarrive with different delays at the mobile terminal 142. Thecommunication paths 130, 132 and 134 may also be differently attenuated.In the downlink, for example, the received signal at the mobile terminal142 may be the sum of differently attenuated communication paths 130,132 and/or 134 that may not be synchronized and that may dynamicallychange. Such a channel may be referred to as a fading multipath channel.

A fading multipath channel may introduce interference but it may alsointroduce diversity and degrees of freedom into the wireless channel.Communication systems with multiple antennas at the base station and/orat the mobile terminal, for example MIMO systems, may be particularlysuited to exploit the characteristics of wireless channels and mayextract large performance gains from a fading multipath channel that mayresult in significantly increased performance with respect to acommunication system with a single antenna at the base station 126 andat the mobile terminal 142, in particular for NLOS signals. Furthermore,Orthogonal Frequency Division Multiplexing (OFDM) systems may besuitable for wireless systems with multipath.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention. Referring toFIG. 13, there is shown a MIMO transmitter 102 and a MIMO receiver 104,and antennas 106, 108, 110, 112, 114 and 116. The MIMO transmitter 102may comprise a processor block 118, a memory block 120, and a signalprocessing block 122. The MIMO receiver 104 may comprise a processorblock 124, a memory block 126, and a signal processing block 128. Thereis also shown a wireless channel comprising communication paths h₁₁,h₁₂, h₂₂, h₂₁, h_(2 NTX), h_(1 NTX), h_(NRX 1), h_(NRX 2), h_(NRX NTX),where h_(mn) may represent a channel coefficient from transmit antenna nto receiver antenna m. There may be N_(TX) transmitter antennas andN_(RX) receiver antennas. There is also shown transmit symbols x₁, x₂and X_(NTX), and receive symbols y₁, y₂ and y_(NRX).

The MIMO transmitter 102 may comprise suitable logic, circuitry and/orcode that may be enabled to generate transmit symbols x_(i) iε{1, 2, . .. N_(TX)} that may be transmitted by the transmit antennas, of which theantennas 106, 108 and 110 may be depicted in FIG. 1B. The processorblock 118 may comprise suitable logic, circuitry, and/or code that maybe enabled to process signals. The memory block 120 may comprisesuitable logic, circuitry, and/or code that may be enabled to storeand/or retrieve information for processing in the MIMO transmitter 102.The signal processing block 122 may comprise suitable logic, circuitryand/or code that may be enabled to process signals, for example inaccordance with one or more MIMO transmission protocols. The MIMOreceiver 104 may comprise suitable logic, circuitry and/or code that maybe enabled to process the receive symbols y_(i) iεE{1, 2, . . . N_(RX)}that may be received by the receive antennas, of which the antennas 112,114 and 116 may be shown in FIG. 1B. The processor block 124 maycomprise suitable logic, circuitry, and/or code that may be enabled toprocess signals. The memory block 126 may comprise suitable logic,circuitry, and/or code that may be enabled to store and/or retrieveinformation for processing in the MIMO receiver 104. The signalprocessing block 128 may comprise suitable logic, circuitry and/or codethat may be enabled to process signals, for example in accordance withone or more MIMO protocols. An input-output relationship between thetransmitted and the received signal in a MIMO system may be written as:y=Hx+nwhere y=[y₁, y₂, . . . y_(NRX)]^(T) may be a column vector with N_(RX)elements, ^(T) may denote a vector transpose, H=[h_(ij)]:iε{1, 2, . . .N_(RX)}; jε{1, 2, . . . N_(TX)} may be a channel matrix of dimensionsN_(RX) by N_(TX), x=[x₁, x₂, . . . x_(NTX)]^(T) is a column vector withN_(TX) elements and n is a column vector of noise samples with N_(RX)elements.

The system diagram in FIG. 1B may illustrate an exemplary multi-antennasystem as it may be utilized in a Universal Mobile TelecommunicationSystem (UMTS) Long-Term Evolution (LTE) system. Over each of the N_(TX)transmit antennas, a symbol stream, for example x₁(t) over antenna 106,may be transmitted. A symbol stream, for example x₁(t), may comprise oneor more symbols, wherein each symbol may be modulated onto a differentsub-carrier. OFDM systems may generally use a relatively large number ofsubcarriers in parallel, for each symbol stream. For example, a symbolstream x₁(t) may comprise symbols on carriers f_(m):mε{1, 2, . . . M},and M may be a subset of the FFT size that may be utilized at thereceiver. For instance, with FFT sizes of N, M<N and may createguard-tones that may allow utilization of variable bandwidth whendeployed. The M sub-carriers may comprise a symbol stream x₁(t), forexample, that may occupy a bandwidth of a few kilohertz to a fewmegahertz. Common bandwidth may be between 1 MHz and up to 100 MHz, forexample. Thus, each symbol stream may comprise one or more sub-carriers,and for each sub-carrier a wireless channel may comprise multipletransmission paths. For example, a wireless channel h₁₂ from transmitantenna 108 to receive antenna 112, as illustrated in the figure, may bemulti-dimensional. In particular, the wireless channel h₁₂ may comprisea temporal impulse response, comprising one or more multipathcomponents. The wireless channel h₁₂ may also comprise a differenttemporal impulse response for each sub-carrier f_(m) of the symbolstream, for example x₂(t). Finally, the wireless channels as illustratedin FIG. 1B depict a spatial dimension of the wireless channel becausethe transmitted signal from each transmit antenna may be receiveddifferently at each receiver antenna. Thus, a channel impulse responsemay be measured and/or estimated for each sub-carrier.

FIG. 2 is an illustration of an exemplary OFDM symbol stream, inaccordance with an embodiment of the invention. Referring to FIG. 2,there is shown a time axis 210; a time domain symbol 0 comprising acyclic prefix CP(0) 202 a, an Inverse Fast Fourier Transform (IFFT)symbol less CP(0) (IFFT(0)) 202 b, and a cyclic prefix CP(0) 202 c; atime domain symbol 1 comprising a cyclic prefix CP(1) 204 a, an IFFTsymbol less CP(1) (IFFT(0)) 204 b, and a cyclic prefix CP(1) 204 c. TheIFFT(0) 202 b and the CP(0) 202 c may together form a complete IFFTsymbol for time domain symbol 0. The CP(0) 202 a may be substantiallysimilar to CP(0) 202 c. Similarly, the IFFT(1) 204 b and the CP(1) 204 cmay together form a complete IFFT symbol for time domain symbol 1, andCP(1) 202 a may be substantially similar to CP(1) 202 c. There is alsoshown an FFT input window 206, a guard time Δt_(g), and a slot marker208. An LTE slot structure, for example, may comprise 3, 6, or 7 OFDMsymbols per slot (two of which may be illustrated in FIG. 2).

To generate an Orthogonal Frequency Division Multiplexing (OFDM) symbol,an output of an IFFT comprising of IFFT(0) 202 b and CP(0) 202 c may beused to generate CP(0) 202 a from CP(0) 202 c, and append it to IFFT(0)202 b. The cyclic prefix CP(0) 202 may be utilized to avoid inter-symbolinterference at an OFDM receiver, in the presence of multi-pathpropagation in the wireless channel.

At an OFDM receiver, for example MIMO receiver 104, a sampled inputsignal may be processed for each received symbol, for example over anFFT input window 206. In order to decode the received symbols, it may bedesirable that the FFT input window 206 may be located in a time domainsymbol time slot, for example in time domain symbol 0. In particular, itmay be desirable that the FFT input window 206 may not extend into aneighboring symbol, to avoid inter-symbol interference. Thus, the slotmarker may indicate the beginning of a slot, for example time domainsymbol slot 0, as illustrated in FIG. 2. The slot marker 208 togetherwith Δt_(g) may define the position of the FFT input window 206 within asymbol slot. In most instances, to keep interference due to themultipath channel as low as possible at the receiver, it may bedesirable to keep Δt_(g) small.

Thus, it may be desirable to acquire symbol level timing, and maintainit as it may drift, for example, because of changes in propagation dueto mobility. In some instances, this may be combined with other timinglevel acquisition and tracking, for example frame synchronization. Inmany instances, synchronization via the Primary Synchronization Signal(PSS) and the Secondary Synchronization Signal (SSS) may be obtained byother means. Symbol timing may be acquired by a timing acquisition andtracking system, which may exploit reference signals (RS) embedded in anOFDM signal. Reference symbols may be known symbols that may betransmitted according to a known pattern over the time, frequency andspatial resources in an OFDM system. In other words, reference symbolsmay be transmitted at known timing instances, on known OFDM carriersover certain antennas. By decoding and processing RS symbols, thereceiver may determine correct timing information, for example, throughcoherent demodulation. RS symbols may be transmitted from each antennain a multiple antenna OFDM system.

In the Enhanced Universal Terrestrial Radio Access (EUTRA) interface, RSsymbols may be generated based on cell-specific hopping pattern, and maycomprise pseudo-noise (PN) covered sequences of Reference symbols. Inaccordance with an embodiment of the invention, the RS tone spacing maybe 6 carriers, per transmit antenna, for example. In accordance withvarious embodiments of the invention, the RS tone spacing may be 2, or 4carriers, for example. The RS sequence may not be known to the mobileterminal (user equipment, UE) during initial acquisition, for examplethrough the synchronization signals. In some instances, after acquiringthe primary synchronization signal (PSS) and the secondarysynchronization signal (SSS), the UE may have obtained the cell-specifichopping pattern for the RS symbols, and the PN covering sequence. Thisinformation may be used to obtain a coarse frame timing. In accordancewith various embodiments of the invention, the RS symbols may then bedecoded in a timing acquisition and tracking block to provide finetiming and tracking. Thus, for example Universal MobileTelecommunication System (UMTS) Long-term evolution (LTE) may employ athree step process for timing recovery. The three step process maycomprise: a) recovering slot timing from the PSS; b) recovering frametiming from the SSS; and c) obtaining an estimation of estimation oftiming offset and tracking from the RS symbols.

FIG. 3A is a diagram of an exemplary OFDM timing acquisition andtracking system, in accordance with an embodiment of the invention.Referring to FIG. 3A, there is shown a common receiver part 342, and atiming part 340. The timing part 340 may comprise an RS phasediscriminator 302, an adder 304, a delay block 306, an integrator 308,and a threshold block 310. There is also shown an RS set input, an errorsignal e_(k), an accumulator signal tt_loop_accum, a threshold inputsignal, a reset control signal reset_cntrl, and an output signalto_accum. The common receiver part 342 may comprise a timing generator312, an RS extraction module or circuit 314, a channel estimation block316, a receiver operations block (RXCVR) 318, a fast Fourier transform(FFT) block 320, a buffering block 330, a sampling bandwidth (BW) filter332, and an analog-to-digital block 334. There is also shown an RFfilter input, a slot timing input from PSS, an RS set output, a to_accumsignal, an rs_strb signal, and a slot_strb signal.

The timing part 340 may comprise suitable logic, circuitry and/or codethat may be enabled to extract timing information by processing an RSset of signals, which may generate an output to_accum that may controlthe timing generator 312, for example. The RS phase discriminator 302may comprise suitable logic, circuitry and/or code that may be enabledto compare the timing of the RS set input signal with, for example, aninput clock signal and may generate an error signal e_(k).

The adder 304 may comprise suitable logic, circuitry and/or code thatmay be enabled to generate a weighted sum signal at its output, from aplurality of inputs. The delay block 306 may comprise suitable logic,circuitry and/or code that may be enabled to delay an input signal by acertain time interval, for example one or more sample periods. The delayblock 306 may comprise suitable logic, circuitry and/or code that may beenabled to delay an input signal by a certain time interval, for exampleone or more sample periods.

The integrator 308 may comprise suitable logic, circuitry and/or codethat may be enabled to generate an output that may be the integration ofone or more input signals, and the integrator 308 may be reset by thereset_cntrl signal. The threshold block 310 may comprise suitable logic,circuitry and/or code that may be enabled to compare a threshold inputsignal with the tt_loop_accum input signal, and generate output signalsreset_cntrl and to_accum. For example, when the tt_loop_accum signal mayexceed the threshold level, the reset_cntrl signal may activate andreset, for example, the integrator 308.

The common receiver part 342 may comprise suitable logic, circuitryand/or code that may be enabled to receive radio frequency signals, andprocess these signals. The processing may comprise FFT computation, RSsymbol extraction, channel estimation and other receiver signalprocessing. The timing generator 312 may comprise suitable logic,circuitry and/or code that may be enabled to generate timing signals forRS extraction, rs_strb, and slot timing, slot_strb. The signal slot_strbmay be used to control FFT timing in the buffering block 330, forexample. The module or circuit 314 may comprise suitable logic,circuitry and/or code that may be enabled to extract the RS symbols fromthe FFT module or circuit 320 output.

The channel estimation module or circuit 316 may comprise suitablelogic, circuitry and/or code that may be enabled to estimate thewireless channel response for RS symbols, which may be desirable forreceiver operations. The receiver operations module or circuit (RXCVR)318 may comprise suitable logic, circuitry and/or code that may beenabled to measure and/or verify performance during receiver operations.The fast Fourier-transform (FFT) module or circuit 320 may comprisesuitable logic, circuitry and/or code that may be enabled to generate aFast Fourier Transform for an input signal. The buffering module orcircuit 330 may comprise suitable logic, circuitry and/or code that maybe enabled to interface with, for example, the FFT engine. The bufferingmodule or circuit 330 may assist in, for example, dedicated processes,measurement processes, multimedia broadcast multicast services (MBMS),and/or SSS processing for hopping pattern determination. In someinstances, each of the processes may be performed in parallel. Thesample BW filter 332 may comprise suitable logic, circuitry and/or codethat may be enabled to filter the signal at its input, and generate anoutput signal with limited bandwidth.

The analog-to-digital (A2D) module or circuit 334 may comprise suitablelogic, circuitry and/or code that may be enabled to receive an analogRF-filtered signal and convert it to a digital signal representation atthe output, with an arbitrary number of bits. The master timer 336 maycomprise suitable logic, circuitry and/or code that may be enabled toprovide basic timing functionality in the receiver. In some instances,the master timer 336 may count over 10 ms periods, and may be clocked at30.72 MHz, for example. The master timer 336 may comprise a slotcounter, and a sample counter. The input to the master timer 336 may beprovided by an operating RF crystal, referred to as a TXCO, for example.

The common receiver part 342 may receive radio frequency signals, andprocess these signals. Processing may comprise FFT computation, RSsymbol extraction, channel estimation and other receiver signalprocessing. Some timing aspects of the common receiver part 342 may becontrolled by the timing part 340. For example, the symbol timing on oneor more receiver subcarrier/carrier frequency, for example f1 and/or f2as illustrated in FIG. 2, may be determined.

The RS phase discriminator 302 may receive at its input, a set of RSsymbols, which may be extracted in the RS extraction module or circuit314 to determine timing information about the RS carrying-carrier. Theoutput of the RS phase discriminator 302 may be communicatively coupledto a first input of the adder 304. A second input of the adder 304 maybe coupled to the output of the delay module or circuit 306, and theoutput of the adder 304 may be a weighted sum of its inputs. The delaymodule or circuit 306 may delay an input signal by a certain timeinterval, for example one or more sample periods. The delay module orcircuit 306 may provide appropriate delay for the feedback signalprovided to the adder 304 from the integrator 308.

An output of the adder 304, tt_loop_accum, may be communicativelycoupled to a first input of the threshold module or circuit 310, and afirst input of the integrator 308. A second input of the thresholdmodule or circuit 310 may be coupled to a threshold-level definingsignal. The threshold module or circuit 310 may compare a thresholdsignal with the tt_loop_accum signal, and generate output signalsreset_cntrl and to_accum. For example, when the tt_loop_accum signal mayexceed the threshold level, the reset_cntrl signal may activate andreset, for example, the integrator 308. In accordance with an embodimentof the invention, the to_accum signal, may increase at a rate that is afunction of Δt, and may thus allow information about Δt to becommunicated to, for example, the timing generator 312, which in turnmay control the FFT input window's position in the time domain. A secondinput to the integrator 308 may be set to a known, constant level, forexample zero. The integrator 308 may integrate one or more inputsignals, and the integrator 308 may be reset by the reset_cntrl signal.

The analog-to-digital (A2D) module or circuit 334 may receive an analogRF-filtered signal and convert it to a digital signal representation atthe output, with an arbitrary number of bits. The A2D 334 output may becommunicatively coupled to an input of the sample BW filter 332. Thesample BW filter 332 may filter the signal at its input, and generate anoutput signal with limited bandwidth and/or attenuate certain frequencybands. The output of the sample BW filter 332 may be communicativelycoupled to a first input of the buffering module or circuit 330. Asecond input to the buffering module or circuit 330 may becommunicatively coupled to the output signal slot from the timinggenerator 312. The buffering module or circuit 330 may interface with,for example, the FFT engine. The buffering module or circuit 330 mayassist in dedicated processes, measurement processes, multimediabroadcast multicast services (MBM), and/or SSS processing for RS PNsequence determination. In some instances, each of the processes may beperformed in parallel. The output of the buffering module or circuit 330may be communicatively coupled to the FFT module or circuit 320.

The FFT module or circuit 320 may generate a Fast Fourier Transform foran input signal communicatively coupled from the buffering module orcircuit 330. Similar to the buffering module or circuit 330, the FFTmodule or circuit 320 may assist in signal processing for dedicatedprocesses, measurement processes, multimedia broadcast multicastservices (MBMS), and/or SSS processing for radio time framing and RS PNsequence determination. A first output of the FFT module or circuit 320may be communicatively coupled to a first input of the RS extractionmodule or circuit 314. The RS extraction module or circuit 314 mayextract the RS symbols from the FFT module or circuit 320 output. Insome instances, it may be desirable to use a generated hopping sequencefrom the demodulated base station signal and/or pseudo-noise (PN)covering for RS decoding. The RS symbols extracted and output at the RSextraction module or circuit 314 may be communicatively coupled to theinput of the timing part 340, and a channel estimation module or circuit316. The hopping pattern may be fed to the RS extraction module orcircuit 314 via the rs_hopping_pattern signal on a second input, asillustrated in FIG. 3A. The RS extraction module or circuit 314 timingmay be controlled via a third input signal rs_strb, communicativelycoupled to an output of the timing generator 312.

The timing generator 312 may generate timing signal for RS extraction,rs_strb, and slot timing, slot_strb. The signal slot_strb may be used tocontrol FFT timing in the buffering module or circuit 330. The timinggenerator 312 may generate the output timing signals for timingcorrections and tracking. The master timer input signal may becommunicatively coupled to the master timer 336 output. The master timer336 may provide basic timing functionality in the receiver. In someinstances, the master timer 336 may count over 10 ms periods, and may beclocked at 30.72 MHz, for example. The master timer 336 may comprise aslot counter, and a sample counter. The input to the master timer 336may be provided by an operating RF crystal, a temperature-controlledcrystal oscillator (TXCO), for example.

The channel estimation module or circuit 316 may estimate the wirelesschannel response for RS symbols, which may be desirable for receiveroperations. The channel estimation output may be communicatively coupledto the RXCVR 318. The RXCVR 318 may measure and/or verify receiverperformance functionality.

FIG. 3B is a diagram illustrating phase offsets for exemplary timingoffsets, in accordance with various embodiments of the invention.Referring to FIG. 3B, there is shown a timing offset plot 350 a, whichmay correspond to Δt_(g)=1; a timing offset plot 350 b, which maycorrespond to Δt_(g)=2; a timing offset plot 350 c, which may correspondto Δt_(g)=3; and a timing offset plot 350 d, which may correspond toΔt_(g)=4.

From PSS and SSS slot markers may be obtained for the received signal towithin the bandwidth of the PSS processing, for example. For the FFT toequalize channel selectivity, it may be desirable that the Cyclic Prefix(CP) may cover the delay spread of the channel. It may also be desirableto minimize the value of Δt_(g) to avoid that the sampled input to FFTprocessing may comprise adjacent symbols (intersymbol interference—ISI)as this may degrade performance. However, Δt_(g) may be tracked, and mayprovide guard against timing shifts due to mobility, for example.

It may be shown that for Δt_(g) unequal to 1, a phase offset may beintroduced into the FFT output, Δt_(g)=1 may correspond to no phaseoffset. This phase offset may be manifested by a complex multiplicativefactor, z(k), applied to a subcarrier, and may be given by the followingrelationship, for a subcarrier k:z(k)=e ^(j(Δtg-1)·(k-1)·2π/N)where N may be the FFT size. When sampling the cyclic prefix, a timingoffset may be manifested by a growing phase offset when indexed by tonefrequency. For a RS tone, a received channel coefficient may be given bythe following relationship:hrx _(k) =Z _(k) ·h _(k) +n _(k)where hrx_(k) may be a received channel coefficient affected by a phaseoffset z(k), h_(k) may be a channel coefficient without phase offset,and n_(k) may be a noise term. In FIG. 3B, various plots may illustratea plurality of z(k), for different values of Δt_(g), where timing offsetplot 350 a may correspond to Δt_(g)=1, timing offset plot 350 b maycorrespond to Δt_(g)=2, timing offset plot 350 c may correspond toΔt_(g)=3, and timing offset plot 350 d may correspond to Δt_(g)=4. Asillustrated in FIG. 3B, timing offset may introduce linearly decreasingphase as a function of frequency.

FIG. 3C is a diagram illustrating phase offsets for exemplary timingoffsets by subcarrier, in accordance with various embodiments of theinvention. Referring to FIG. 3C, there is shown a timing offset plot 352a, which may correspond to Δt_(g)=0; a timing offset plot 352 b, whichmay correspond to Δt_(g)=1 for subcarrier 512; a timing offset plot 352c, which may correspond to Δt_(g)=2 for subcarrier 511; and a timingoffset plot 350 d, which may correspond to Δt_(g)=3 for subcarrier 510.

For Δt_(g)>0, the FFT may be sampled late, and a sampling timingadjustment may be desirable. As illustrated in FIG. 3C, for latesampling, timing offset may manifest phase offset across the bandwidthof the channel, as illustrated for a plurality of subcarriers by theplots 352 a-352 d. In some instances, the slope of the phase offset infunction of the frequency may be linear with a positive slope for earlysampled FFT, and a negative slope for late sampled FFTs, for example. Anexemplary phase detector that may use adjacent RS tones per OFDM symbolmay be given by the following relationship:

$e_{k} = {\frac{1}{N_{t}} \cdot {\sum\limits_{{{all}\mspace{14mu}{{tones}{(l)}}},{{all}\mspace{14mu}{antennas}}}{{imag}( {{hrx}_{l} \cdot {hrx}_{l + 6}^{*}} )}}}$where a phase change between hrx_(i) and hrx_(i+6) (another RS tone,spaced 6 subcarriers apart, for example), may be averaged over alldesirable tones, and all desirable antennas, which may be N, terms.

FIG. 4 is a flow chart illustrating an exemplary timing acquisition andtracking process, in accordance with an embodiment of the invention.After initialization in step 402, the primary and secondarysynchronization signals, (PSS) and (SSS), may be decoded in step 404.The decoding of the PSS and SSS may provide coarse timing informationfor frame and slot synchronization, for example. Using the timinginformation, the FFT block 320, for example, may generate an FFT of areceived signal. From this FFT, in step 406, the RS extraction block314, for example, may extract an RS set in the RS extraction block 314.This RS set may be fed to the timing part 340. In step 408, the timingpart 340 may track Δt_(g) using the feedback loop depicted in FIG. 3A.Since the output signal to_accum may be generated as a function ofΔt_(g), the to_accum output signal from the threshold block 310 maycarry information about Δt_(g) to the timing generator 312. Thus, theoutput signal to_accum may enable tracking of the symbol timing, andadjust the timing of the receiver, for example in the timing generator312. In step 410, the timing generator 312 may adjust the timing basedon the input signal to_accum. The adjusted and tracked timing may becommunicatively coupled from the timing generator 312 to the bufferingblock 330, where the timing of the FFT input window may be adjustedcorrespondingly.

In accordance with an embodiment of the invention, a method and systemfor a RS timing loop for OFDM symbol synchronization and tracking maycomprise tracking symbol timing in an Orthogonal Frequency DivisionMultiplexing (OFDM) signal based on at least a reference symbol set, asdescribed for FIG. 2 and FIG. 3A. A receiver timing, as described forFIG. 2 and FIG. 3A, may be adjusted based on at least the symbol timing.

The symbol timing may be tracked by generating an output signal as afunction of a guard time Δt_(g) in a phase discrimination feedback loop,for example timing part 340. The reference symbol (RS) set may begenerated in an RS extraction module or circuit 314, from at least afast Fourier transform of the received OFDM signal. The receiver timingmay be coarsely adjusted and then finely adjusted, for example. Thecoarse receiver timing adjustment may be based on processing at least aprimary synchronization signal and a secondary synchronization signal,as described for FIG. 3A. The reference symbol set may comprise aplurality of time-frequency slots. The plurality of time-frequency slotsmay change according to a frequency shift and pseudo-noise sequence thatmodulates the reference symbols. The OFDM signal may conform to aUniversal Mobile Telecommunications Standards (UMTS) long-term evolution(LTE) signal. The adjustment of the receiver timing may be controlledvia a receiver timing generator 312, for example.

Another embodiment of the invention may provide a machine-readableand/or computer-readable storage and/or medium, having stored thereon, amachine code and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for a methodand system for a reference signal (RS) timing loop for OFDM symbolsynchronization and tracking.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments failing within the scope of the appended claims.

What is claimed is:
 1. A wireless device, comprising: a reference signalphase discriminator configured to compare a reference signal timingembedded within a signal received from a base station to a referenceclock signal timing to provide an error signal; a feedback moduleconfigured to receive the error signal based on an average phase changeof a plurality of reference symbols and to provide an accumulatorsignal, each reference symbol from among the plurality of referencesymbols having a corresponding symbol guard time; a threshold blockconfigured to receive the accumulator signal and to provide an outputsignal that varies as a function of an average symbol guard time of theplurality of reference symbols; and a timing generator configured togenerate a timing signal based on the output signal.
 2. The wirelessdevice of claim 1, wherein the average phase change is an average phasechange of the plurality of reference symbols corresponding to aplurality of subcarriers.
 3. The wireless device of claim 1, wherein theaverage phase change is an average phase change of the plurality ofreference symbols transmitted or received through a plurality ofantennas.
 4. The wireless device of claim 1, wherein the error signal isfrom among a plurality of successive error signals, and wherein thefeedback module comprises: an integrator configured to integrate theplurality of successive error signals over a period of time to providean integrated error signal.
 5. The wireless device of claim 4, whereinthe feedback module further comprises: an adder; and a delay blockcoupled to the integrator and to the adder, the delay block configuredto delay the transfer of the integrated error signal to the adder. 6.The wireless device of claim 5, wherein the adder is configured toprovide a weighted sum of the error signal and the integrated errorsignal as the accumulator signal.
 7. The wireless device of claim 1,further comprising: a reference symbol extraction module configured toextract a reference symbol utilizing a fast Fourier transform (FFT)sample window based on the timing signal to provide an extractedreference symbol.
 8. The wireless device of claim 7, wherein thereference symbol is from among a plurality of reference symbols, furthercomprising: a reference symbol phase discriminator configured to receivethe plurality of reference symbols to provide the error signal, andwherein the extracted reference symbol is received by the referencesymbol phase discriminator.
 9. A method in a wireless device,comprising: comparing a plurality of received reference symbols, eachreceived reference symbol from among the plurality of received referencesymbols having a corresponding symbol guard time, to a reference clocksignal; generating a plurality of error signals based on the comparison;generating an output signal that varies as a function of the pluralityof reference symbol guard times based on the plurality of error signals;and adjusting a receiver timing based on the output signal.
 10. Themethod of claim 9, wherein the comparing comprises: comparing theplurality of received reference symbols corresponding to a plurality ofsubcarriers.
 11. The method of claim 9, wherein the generating theplurality of error signals comprises: generating a plurality ofsuccessively received error signals as the plurality of error signals.12. The method of claim 11, wherein the generating the plurality oferror signals comprises: generating a weighted sum of error signals byadding the plurality of error signals and a plurality of previouslyreceived error signals.
 13. the method of claim 9, wherein the adjustingcomprises: adjusting a fast Fourier transform (FFT) sample window basedon the output signal.
 14. The method of claim 9, wherein the comparingcomprises: comparing a plurality of modulated received reference symbolshaving a plurality of time-frequency slots modulated according to afrequency shift and pseudo-noise (PN) sequence.
 15. A method, in awireless device, comprising: decoding a primary synchronization signal(PSS) and a secondary synchronization signal (SSS) from a received dataframe to provide a timing information; extracting a reference symbolfrom the received data frame based on the timing information;determining a guard time corresponding to the reference symbol based ona comparison of the reference symbol to a reference clock; and adjustinga receiver timing based on the guard time.
 16. The method of claim 15,wherein the extracting comprises: extracting the reference symbolutilizing a fast Fourier transform (FFT) sampling window.
 17. The methodof claim 16, wherein the adjusting further comprises: adjusting the FFTsampling window based on the guard time.
 18. The method of claim 15,wherein the decoding comprises: decoding the PSS and the SSS from thereference symbol.
 19. The method of claim 15, wherein the received dataframe is from among a plurality of received data frames, the methodfurther comprising: repeating the decoding through the adjusting foreach data frame from among the plurality of data frames.
 20. The methodof claim 19, wherein the plurality of received data frames are receivedsuccessively, and wherein the determining further comprises: determiningthe guard time as a weighted sum of a plurality of guard timescorresponding to the plurality of received data frames by iterativelyadding one received data frame from among the plurality of received dataframes to another, previously received data frame, from among theplurality of received data frames.